Engine ignition systems may include a spark plug for delivering an electric current to a combustion chamber of a spark-ignited engine to ignite an air-fuel mixture and initiate combustion. Based on engine operating conditions, spark plug fouling can occur wherein a firing tip of the spark plug insulator becomes coated with a foreign substance, such as fuel, oil, or soot. Once fouled, the spark plug may be unable to provide adequate voltage to trigger cylinder combustion until the spark plug is sufficiently cleaned or replaced. For example, the spark plug may be cleaned by burning off the soot accumulated on the fouled spark plug by operating the engine in speed-load conditions that sufficiently raise the spark plug tip temperature.
In modern boosted engine systems, which have a large dynamic operating range, a heat range for spark plugs is selected to avoid abnormal combustion events, such as pre-ignition, at wide open throttle and high engine speed conditions. In particular, the spark plugs selected are typically a low heat range plug to avoid pre-ignition at high engine speed and load conditions. Consequently, even if engine spends more time at high speed and load conditions than a normally aspirated engine, the spark plugs still do not get warm enough to trigger pre-ignition. However, for the same reason, the spark plugs also do not get warm enough to burn off soot deposits. Other attempts to address the spark plug fouling issue include advancing spark timing to burn off the deposits over the spark plug. One example approach is shown by Yamada et al. in U.S. Pat. No. 6,505,605. Therein, in response to spark plug fouling, spark timing is advanced for a number of combustion events, the amount of spark advance supplied adjusted based on an indicated mean effective pressure of the cylinder. In addition, to reduce further spark plug fouling, fueling to the affected cylinder is adjusted to provide a leaner air-fuel mixture. The spark timing advance and fueling adjustment is maintained until soot is burned off from the fouled spark plug, after which nominal fueling and spark timing is resumed.
However, the inventors herein have recognized potential issues with such approaches. As one example, the continued advancement of spark timing in '605 may induce engine knocking. That is, if spark timing is maintained advanced to raise the spark plug tip temperature and burn off the soot, the elevated temperature may trigger cylinder knock events. On the other hand, if spark timing is not sufficiently advanced, and the spark plug tip temperature not sufficiently raised, the spark plug may remain soot fouled, leading to cylinder misfire events. In addition, the engine controller may be conflicted between advancing spark timing to address spark plug fouling and retarding spark timing to address knock. Further still, the prolonged advancing of spark timing may affect engine fuel efficiency and exhaust emissions.
In one example, the issues described above may be at least partly addressed by a method for addressing spark plug fouling by periodically advancing spark timing and interspersing bursts of spark timing advance with operation at nominal spark timing. One example method comprises: supplying a first advanced spark timing to a cylinder for a first single combustion event; supplying a nominal spark timing to the cylinder for a predetermined number of combustion events after supplying the first advanced spark timing to the cylinder; and supplying a second advanced spark timing to the cylinder for a second single combustion cycle after the predetermined number of combustion events. In this way, soot deposits on a fouled spark plug may be burnt off with reduced propensity for inducing knock.
In one example, spark plug soot fouling may be determined based on the output of an ionization sensor. In response to the indication of spark plug fouling, a first advanced spark timing may be supplied to the affected cylinder for at least one combustion cycle. For example, the degree of spark timing advance applied and the number of combustion events the first spark timing advance is applied for may be adjusted to sufficiently raise the spark plug tip temperature to a temperature that enables soot burn-off. For example, spark timing may be advanced for only a single combustion event. Then, spark timing may be returned to a nominal spark timing (e.g., MBT or borderline spark timing) for a predetermined number of combustion events. The predetermined number may be adjusted to prevent overheating of the spark plug tip temperature, and induction of knock events. Then, a second advanced spark timing may be supplied to the cylinder for one or more combustion events. Herein, the second advanced spark timing may be adjusted (e.g., in real-time) based on ionization sensor output following the applying of the first advanced spark timing (and the nominal timing). For example, based on the ionization sensor output, a soot load remaining on the fouled spark plug may be inferred and a margin to knock may be determined. A degree of spark timing advance applied and the number of combustion events the second spark timing advance is applied for may then be adjusted to burn off the remaining soot while reducing a propensity, and intensity, of knock. The spark timing may then be returned to the nominal spark timing. In one example, since a portion of the soot load may have been burned off when the first advanced spark timing is applied, the second advanced spark timing may be less advanced than the first advanced spark timing.
The technical effect of supplying a nominal spark timing in-between the applying of a first and a second advanced spark timing is that overheating of the spark plug may be reduced. As such, this reduces the likelihood of knock being induced when using spark timing advance to burn off soot accumulated on a fouled spark plug. By using periodic advanced spark events with intermediate nominal events, spark plug tip temperatures may be raised sufficiently high to burn off soot (and address spark plug fouling) but not raised so high that overheating issues arise, such as knock, NVH, and durability constraints. By improving the engine's knock margin, the amount of spark timing advance that may be applied to burn off the soot can also be enhanced. By improving spark plug fouling control, the likelihood of misfires is reduced. In addition, spark plug temperatures may be controlled, reducing the likelihood of knock events. Overall, spark plug health and life can be improved.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.